The marine CSEM (Controlled Source Electromagnetic) exploration method uses a man-made source to generate Electromagnetic (EM) waves and deploys receivers on the seafloor (if the survey is in a marine environment) to record EM signals. The recorded EM signals are analyzed to infer sub seafloor structures and/or determine the nature of particular structures, such as reservoirs. FIG. 1 shows the principle of marine CSEM exploration with a Horizontal Electric Dipole (HED) source 11 (depicted with a receiver 13 resting on the seafloor 12). U.S. Pat. No. 6,603,313 is an example of a published disclosure of a CSEM technique for hydrocarbon exploration.
At present, receivers are deployed by free fall to the seafloor or lowered on a winch. In either case, the final orientation of the receiver on the seafloor is unknown. Receiver orientations are required to determine the three-dimensional EM field vectors measured at receiver locations. The measured fields are then decomposed into components in preferred directions (for example, inline, cross-line, and vertical) for analysis, inversion and interpretation. Inaccurate receiver orientations can have significant effects on the decomposed components. Therefore the determination of receiver orientations could significantly affect data interpretation. The present invention provides techniques to determine receiver orientations using independent (non-EM) observations.
In order to completely measure three-dimensional EM fields, receivers need to be equipped with three mutually-perpendicular antennas for electric fields and three mutually-perpendicular magnetic sensors for magnetic fields. Three angles are necessary and sufficient to uniquely define the receiver orientations. These three angles establish the relationships between the measurement coordinates and receiver coordinates. A number of methods can be used to define the receiver orientations in the measurement coordinates. They are equivalent and can be converted from one to another. One way to define the receiver orientations is using azimuth and tilts for two horizontal channels, as indicated in FIG. 2. In FIG. 2, (X, Y, Z) are assumed to be the measurement coordinates with X directed to the geodetic east, Y to the geodetic north, and Z upward. (X′″, Y′″, Z′″) are the receiver coordinates, i.e., are the coordinate system defined by the receiver's antennas, and can be designated the “east”, “north” and vertical channels. (X′, Y′, Z′) and (X″, Y″, Z″) are auxiliary coordinates used in the transformation between (X, Y, Z) and (X′″, Y′″, Z′″). X′ is the projection of X′″ on the horizontal plane XY, while Y′ is the projection of Y′″. With these definitions, the receiver azimuth (α) is defined as the angle between Y and Y′, the east channel tilt (β) is the angle between X′ and X′″, the north channel tilt (γ) is the angle between Y′ and Y′″.
Current EM receiver designs include long horizontal arms with electrodes attached to the ends. Two opposing electrodes form a dipole, with typical dipole lengths ranging between eight (8) and ten (10) meters. The length of the horizontal dipole is dictated by the design signal-to-noise floor. For a given electrode-amplifier combination, the signal-to-noise floor will decrease as the dipole length increases. While there is an obvious benefit for ever increasing horizontal dipole lengths, the receiver package must remain physically manageable, with ease of deployment and recovery being important design considerations. Further, the EM receiver package must remain stationary on the seafloor during data acquisition. Any vibration induced motion (through seafloor currents) will be interpreted as noise. To minimize vibration induced motion, the EM receiver's gravity base must be in direct contact with the seafloor. The horizontal arms must be flexible to follow the general topography of the seafloor, as illustrated in FIG. 3A. Rigid horizontal arms could result in the EM receiver being suspended across topographical features and being subject to significantly reduced signal-to-noise ratios, as illustrated in FIG. 3B.
Current horizontal arms are designed with symmetrical cross-sections, typically circular. The arms are equally flexible in both the horizontal and vertical planes. The flex in the horizontal arm typically may be approximately 5 degrees. It is straightforward to mount a three-dimensional attitude sensor (azimuth, α′ and two-dimensional tilt, β′ and γ′) on the EM receiver frame. Due to the flexible horizontal arms, α′≠α, β′≠β and γ′≠γ. The differences between the observed and desired quantities will represent a bias for a particular EM receiver deployment. Further, the bias will vary between receivers and between deployments. Asymmetrical cross-section designs (including elliptical and rectangular) could increase the rigidity in the horizontal plane (α′α), but the vertical flexibility will remain (β′≠β and γ′≠γ). The presence of these angular biases has resulted in EM receiver orientations being estimated from CSEM data and not from direct observations. Thus, because CSEM receiver arms need to be flexible, an attitude sensor mounted on the receiver frame is insufficient for determining accurate orientations of the dipole axes.
One common method for estimating receiver orientations from CSEM data is polarization analysis. The method, originally proposed by Smith and Ward (“On the computation of polarization ellipse parameters,” Geophysics 39, 867-869 (1974)) is based on the fact that the EM field amplitude of the signal recorded by a receiver is maximized when the receiver dipole is in the direction of the transmitter (i.e. the major axis of the polarization ellipse) provided the transmitter is towed directly towards the receiver. Polarization analysis was the main method to determine receiver azimuth in data processing of early marine CSEM applications. The method requires at least one towline be towed directly over each receiver. Receiver azimuth accuracy provided by this method is not very high. The polarization technique is robust partly because it is insensitive to small errors in the orientation angle. Unfortunately the cross-line component is highly sensitive to small orientation errors. The average error in receiver azimuths is larger than 5 degrees from a vessel with a dynamic positioning system. It could be significantly worse using a vessel without dynamic positioning in rough weather conditions, where the source may not be towed directly over the receiver (inducing either an across-line offset or a yaw between the source transmitter and the sail line). Small errors (<10°) in receiver orientation have a negligible effect on the in-line data, but a pronounced effect on the cross-line data, with increased amplitude (Amp Ey) and improved coherency in cross-line phase (Phase Ey).
Behrens uses coherency and correlation in natural EM signals between receivers to determine relative azimuth. (“The Detection of Electrical Anisotropy in 35 Ma Pacific Lithosphere: Results from a Marine Controlled-Source Electromagnetic Survey and Implications for Hydration of the Upper Mantle,” Ph.D. Thesis, University of California, San Diego (2005)). This method was developed for receivers without towing a towline to complement the polarization analysis. The method determines the relative azimuth angle between two receivers. In order to find the receiver's absolute azimuth, the method requires the azimuth of the reference receiver to be known. Success in using this method is dependent on whether high quality natural signals are recorded by both receivers. The accuracy of this method is normally lower than the polarization analysis.
R. Mittet et al. (“E020: Inversion of SBL data acquired in shallow waters,” EAGE 66th Conference & Exhibition—Paris, France, Jun. 7-10 (2004)) used inversion to determine receiver azimuth. This method overcomes limitations in both the polarization analysis and correlation method by using natural EM signals.
In PCT Patent Publication No. WO 2004/049008 (Electromagnetic Surveying for Hydrocarbon Reservoirs), the inventors state that marine CSEM receiver data are “resolved along a direction perpendicular to a line connecting the source location and the detector location and in a horizontal plane”. The publication does not appear to discuss how the detector's attitude is estimated relative to a horizontal plane. The publication further states, “The components of the detected electric field along these directions is determined from the angular orientation of the orthogonal dipole antennae comprising the detector antenna relative to the line joining the source location and the detector location. This can be easily determined using standard instrumentation, such as, for example, active or passive sonar to determine the relative positions of the source location and the detector location, and a magnetic compass to determine the detector antenna orientation”. The use of a magnetic compass will estimate the azimuth (α′) but not the tilts (β and γ). Moreover, the azimuth that the compass measures will be that of the receiver frame (α′), which will differ from that of the dipole (α). Further, this publication does not appear to acknowledge the possibility that the arms are not orthogonal in the horizontal plane.
Thus, widely used methods focus on the receiver azimuth (α′), but do not appear to disclose how to determine receiver orientations uniquely, i.e. both the azimuth (α) and the tilts (β and γ) of the two horizontal channels. The reasons for neglecting the receiver's other two angles may be:                1) Data interpretation is mainly focused on the inline electric component, which is normally not significantly affected by the tilt angles if the seafloor topography is minimal;        2) The vertical electric component is not measured or not fully utilized in data interpretation; and        3) No reliable and accurate data driven method is available to determine the receiver orientations.        
The two tilts (β and γ) are normally small (<10 degrees). In order to determine them, it requires that the method itself must fully model the field source and receiver geometry and the acquisition system must accurately record the geometry.
Effects of receiver orientations on the three electric components can be seen in FIGS. 4 through 6. The source and receiver geometry used in this modeling is taken from a field survey. The resistivity model is a layered earth model with a water depth of 125 meters. The towline direction is 265.57 degrees from the geodetic north, clockwise. In the modeling, the receiver (with azimuth α in FIG. 2) misalignment (δα) with the towline is 15 degrees, the inline dipole tilts (β) up 5 degrees, and the cross-line dipole tilts (γ) down 3 degrees. The receiver misalignment (δα) simulates the misalignment between the dipole (α) and receiver frame (α′). The modeling frequency is 0.25 Hz. In each of these three drawings, the solid line represents an aligned and level receiver, the circles a level receiver with misalignment δα=15°, the + symbols an aligned receiver with tilts of β=5° and γ=−3°, and the broken line a misaligned and tilted receiver. FIG. 4 shows the effect of receiver orientations on the inline electric field component, FIG. 5 the crossline electric field component, and FIG. 6 the vertical electric field component. Compared with the ideal situation (a level receiver aligned with the towline), these figures show that the azimuth has a much bigger effect on the two horizontal channels than on the vertical component, especially on the cross component; while the tilts have more effects on the vertical component. These effects can be significant, for example, about one order of magnitude for the cross and vertical components of this example (FIGS. 5 and 6). The example clearly demonstrates the importance of determining all three angles. Receiver azimuth alone cannot uniquely define the orientation of an EM receiver deployed on the seafloor.
Lu (U.S. Provisional Patent Application No. 60/701,817) uses inversion to estimate the receiver's three-dimensional orientations, which are considered parameters in the inversion process. This approach requires a resistivity model, which should be created as close to the truth as possible to make the inversion converge quickly and to avoid a local minimum solution for the inversion.
In summary, a method is needed for determining an EM receiver's three-dimensional orientation without any limitations on transmitter-receiver geometry or any a-priori knowledge of an underlying resistivity model. This invention satisfies this need by estimating the three-dimensional orientation through indirect observations.